AD845

REV. E

–7–

MEASURING AD845 SETTLING TIME

Figure 1 shows AD845 settling time performance. This measure-

ment was accomplished by driving the amplifier in the unity

gain inverting mode with a fast pulse generator. The input

summing junction was measured using false nulling techniques.

Settling time is defined as the interval of time from the application

of an ideal step function input until the closed-loop amplifier

output has entered and remains within a specified error band.

Components of settling time include:

1. Propagation time through the amplifier

2. Slewing time to approach the final output value

3. Recovery time from overload associated with the slewing

4. Linear settling to within a specified error band

These individual components can be seen easily in Figure 1.

Settling time is extremely important in high speed applications

where the current output of a DAC must be converted to a

voltage. When driving a 500

W load in parallel with a 100 pF

capacitor, the AD845 settles to 0.1% in 250 ns and to 0.01% in

310 ns.

Figure 1. Settling Characteristics 0 V to 10 V Step

Upper Trace: Output of AD845 Under Test (5 V/Div)

Lower Trace: Error Voltage (1 mV/Div)

Figure 2. Settling Time Test Circuit

A HIGH SPEED INSTRUMENTATION AMP

The 3-op amp instrumentation amplifier circuit shown in

Figure 3 can provide a range of gains from unity up to 1000 and

higher. The instrumentation amplifier configuration features

high common-mode rejection, balanced differential inputs, and

stable, accurately defined gain. Low input bias currents and fast

settling are achieved with the FET input AD845.

Most monolithic instrumentation amplifiers do not have the

high frequency performance of the circuit in Figure 3. The cir-

cuit bandwidth is 10.9 MHz at a gain of 1 and 8.8 MHz at a

gain of 10; settling time for the entire circuit is 900 ns to 0.01%

for a 10 V step (Gain = 10).

The capacitors employed in this circuit greatly improve the

amplifier’s settling time and phase margin.

Figure 3. High Performance, High Speed Instrumentation

Amplifier

Table I. Performance Summary for the 3-Op Amp

Instrumentation Amplifier Circuit

3-Op Amp In-Amp

Small Signal

Settling Time

Gain

Bandwidth

to 0.01%

1Open

10.9 MHz

500 ns

22 k

W

8.8 MHz

500 ns

10

226

W

2.6 MHz

900 ns

100

20

W

290 kHz

7.5

ms

Note: Resistors around the amplifiers’ input pins need to be small enough in

value so that the RC time constant they form, with stray circuit capacitance,

does not reduce circuit bandwidth.

Figure 4. The Pulse Response of the 3-Op Amp

Instrumentation Amplifier. Gain = 1, Horizontal Scale =

0.5 ms/Div and Vertical Scale = 5 V/Div.